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Bio-inspired Crystallization

Nature demonstrates that it is possible to produce minerals with specific sizes, remarkable complex morphologies, hierarchical composite structures and orientations under ambient and environmentally-friendly reaction conditions.

As organisms clearly cannot manipulate parameters such as temperature or pressure, as is commonly done synthetically to control crystal growth, the strategies they use commonly rely on organic molecules to control mineralisation. These can be in the form of an insoluble organic matrix, which generates a unique environment in which crystallisation occurs and can influence nucleation processes. Soluble organic additives are also typically present during crystal growth, and can influence crystal texture and morphology. These processes can in turn provide inspiration for synthetic crystal growth experiments.

A variety of strategies are commonly used to control the properties of crystals grown in synthetic systems, which have been developed using biological mechanisms as an inspiration. There is significant interest in the use of soluble additives to control crystallization. These can vary from small molecules to large polymers, and can generate crystals with complex morphologies and with composite structures in which particles of one phase are embedded within the matrix of another and showing superior mechanical properties. Soluble additives are also believed to be key in controlling crystal polymorphs. Indeed, crystallization in biological systems often proceeds via amorphous precursor phases, and study and exploitation of this mechanism is being carried out to gain greater control over the crystallization process and to use amorphous phases as novel synthetic materials.

Single crystal of calcite with complex morphology grown in a colloidal crystal template

Insoluble organic matrices are also extremely important in controlling the formation of biominerals. At the most fundamental level, a key feature of biological systems is that crystallisation always occurs within compartments. This can significantly affect the progress and outcome of reactions occurring within, and crystallization is no exception to this. A range of different model systems including including synthetic vesicles, porous glasses, the pores in track etch membranes, droplet arrays and droplets in microfluidic systems have therefore been used to study the effects of confinement of crystal nucleation and growth from the nanometer to the micron scale. 

These studies demonstrate that features such as morphology, orientation and polymorph can all be controlled using confinement. Finally, organic matrices, such as self-assembled monolayers and Langmuir monolayers can also be used to direct crystal growth, leading to control of the nucleation face and sometimes an epitaxial relationship between the matrix and the crystal.

Calcium carbonate grown in the presence of polymer additives